What we want to happen is that upon making a request to our sites, the following happens:
If the user doesn't have the PHP_ENGINE cookie set, set it. There is a `wgPHP7percent` chance that the cookie will route all subsequent requests for the same user to be rendered by php7 for a fixed amount of time (7 days seems sensible?)
An additional possible measure is:
- If the request is uncachable and has no PHP_ENGINE cookie set, make the chance extraction in vcl and send the request to the appropriate backend. Add the cookie in the backend response to try to persist the choice
The first part of this is analogous to what we've done for the HHVM transition in https://gerrit.wikimedia.org/r/c/mediawiki/extensions/NavigationTiming/+/164984 - with possibly two differences:
# I'd like to include logged-in users in the extraction (or at least allow it)
# We'll add the code to WikimediaEvents, which looks like the right place for this.